1. Field of the Invention
The present invention relates to a dead-time compensation apparatus of a PWM inverter and method thereof, and more particularly to a dead-time compensation apparatus of a PWM inverter and method thereof that produces a dead-time compensation voltage by using a lookup table.
2. Description of Prior Art
The commonly used technologies of controlling the inverter can be divided into two parts: scalar control and vector control. Although the scalar control is inferior at dynamic response, speed control ratio, and control accuracy in comparison with the vector control, the scalar control is simple in structure, easy to implement, and stable to control. Hence, the scalar control is widely used in a non-servo control system. The scalar control is also called a constant V/f control, or a variable voltage variable frequency control (VVVF control) method. Generally speaking, the constant V/f control is an open-loop control, namely, a rotational speed of the induction motor is easily changed by using an inverter without feeding back the rotational speed. However, torque of the induction motor will reduce because output frequency of the inverter increases while input voltage of the inverter is not simultaneously changed. Hence, in order to keep magnetic flux of the induction motor constant to generate maximum efficiency, the ratio of voltage magnitude to operation frequency has to be constant.
When the induction motor is operated at low speed and light load condition, output voltage error of the inverter is significant due to the voltage drop of the switching elements. Hence, performance of the motor control for low speed and light load operation is lower even through the constant V/f control is easy to implement.
In addition, due to non-linearity in a solid-state switching element, such as turn-on delay and turn-off delay, the solid-state switching element does not immediately turn-on or turn-off when being driven by an input trigger command. In order to avoid both top and bottom side switching elements turning on or turning off simultaneously, a short delay time or so-called dead-time has to be added.
A strategy of the dead-time compensation is to add a time delay when the switching element is switched from a turn-off state to a turn-on state. Also, the time delay is determined according to switching speed of the switching element. However, the fundamental components of the output voltage of the inverter reduce and the low-frequency harmonic components increase when the dead-time is added. Hence, the low-frequency harmonic components could significantly influence the motor to cause current distortion in the zero-crossing current region when the motor is operated at an open-loop low-speed condition.
Reference is made to FIG. 1A which is a block diagram of a prior art dead-time circuit of an inverter. This is one of the most commonly used dead-time compensation approaches. A dead-time compensation voltage is produced by calculating a three-phase output current detected from a motor 30A. Namely, a current detecting circuit 40A is used to detect the three-phase output current of the inverter 20A. The three-phase output current is received by a dead-time compensation module 50A. According to the polarity of the three-phase current, a PWM voltage command adds or subtracts an offset voltage to produce a trapezoid compensation curve in phase with the output current. However, the ideal compensation voltage is not easily obtained due to the non-smoothing trapezoid compensation curve. More particularly, the distortion of output current could cause a discontinuous motion of the induction motor when the induction motor is operated at low speed and light load condition.
Another of the most commonly used approaches (voltage feedback) is described as follows and the demonstrated approach also improves the above-mentioned the current distortion when the motor is operated at an open-loop low-speed condition.
Reference is made to FIG. 1B which is a block diagram of a prior art dead-time circuit of an inverter. The difference between the approach and the above-mentioned approach is that a voltage detecting circuit 60A is further added. The voltage detecting circuit 60A is used to detect an offset voltage of the three-phase output voltage of the inverter 20A. Hence, a dead-time compensation voltage can be calculated according to an offset voltage and the polarity of the three-phase current. Also, the curve of the output current is similarly to a smoothing sinusoidal waveform to provide more accurate dead-time compensation voltage to improve current distortion when the motor is operated at low speed and light load condition. However, a main disadvantage of this approach is that the voltage detecting circuit 60A increases hardware costs.
Accordingly, a dead-time compensation apparatus of a PWM inverter and a method thereof are provided to obtain a more accurate dead-time compensation voltage without increasing hardware costs and improve the efficiency of operating the induction motor at low speed and light load condition.